feature #18118 Add note about custom port being ignored by specific mailer transport DSN (dreadnip)

This PR was submitted for the 6.3 branch but it was merged into the 5.4 branch instead.

Discussion
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Add note about custom port being ignored by specific mailer transport DSN

See symfony/symfony#49768

Seems like there has been some confusion about this:
* symfony/symfony#46979
* symfony/symfony#44794

People start with the default they're given by the docs, e.g. `sendgrid+smtp` and then try to append `:<port>` to configure it, which doesn't work.

As far as I know, the official recommendation is to use the regular `smtp` transport instead when using custom config: symfony/symfony#36224 (comment)

Commits
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b39027f Add note about custom port configuration being ignored by specific mailer transport DSN

Author: fabpot

mailer.rst

@@ -236,6 +236,17 @@ OhMySMTP             ohmysmtp+smtp://API_TOKEN@default                    n/a
 
     Note that the protocol is *always* HTTPs and cannot be changed.
 
+.. note::
+
+    The specific transports, e.g. ``mailgun+smtp`` are designed to work without any manual configuration.
+    Changing the port by appending it to your DSN is not supported for any of these ``<provider>+smtp` transports.
+    If you need to change the port, use the ``smtp`` transport instead, like so:
+
+    .. code-block:: env
+
+        # .env
+        MAILER_DSN=smtp://KEY:DOMAIN@smtp.eu.mailgun.org.com:25
+
 High Availability
 ~~~~~~~~~~~~~~~~~